1. Field of the Invention
This invention relates to a control system and a control method of an electric motor for controlling a torsion system containing an angle transmission error of a reduction gear unit used with a robot, etc.
2. Description of the Related Art
Angle transmission error correction of a reduction gear unit 6 in a torsion system containing an angle transmission error of a reduction gear unit used with a robot, etc., in prior art will be discussed. As shown in a block diagram of FIG. 10, motor current Im controlled by control means 1 is converted into torque Kt.multidot.Im in a torque constant term 9 in a motor 2 and the torque Kt.multidot.Im is converted into speed .omega.m in a motor dynamics term 10 expressed by the reciprocal of the sum of motor shaft inertia Jm and motor shaft viscous resistance Dm. The speed .omega.m is converted into motor turning angle .theta.m in an integration term 11. Further, the motor turning angle .theta.m is multiplied by reduction gear ratio 1/Rg in the reduction gear unit 6 and the result is converted into output angle .theta.g to which an angle transmission error is added, then the conversion result is output. By the way, as shown in FIG. 11, the reduction gear unit 6 and a load 8 are connected by a spring component 7, thus torsion angle .theta.s of the difference between the output angle .theta.g of the reduction gear unit 6 and load position .theta.L of the load 8 occurs. Further, the torsion angle .theta.s is multiplied by spring constant Ks in a spring constant term 14 and is converted into a torsion torque. This torsion torque acts on the load 8 as an action force and is converted into speed .omega.L in a load dynamics term 15 expressed by the reciprocal of the sum of load inertia JL and load shaft viscous resistance DL, then the speed .omega.L is output in an integration term 16 as the load position .theta.L. Torsion torque also occurs in the motor 2 as a reaction force. Thus, control of the reduction gear unit 6 in the control system in the prior art is only to execute conversion to the output angle .theta.g by multiplying the motor turning angle .theta.m by the reduction gear ratio 1/Rg; an angle transmission error occurring at the time is not clarified. For example, as shown in Japanese Patent Laid-Open No. Sho 62-155348, disturbance torque appearing at motor shaft end is all assumed to be an angle transmission error of a reduction gear unit 6 and such a correction signal to cancel the disturbance torque is generated by correction means 30 and is added to a speed command and a torque command, thereby correcting the angle transmission error for improving load tip locus precision. Alternatively, as shown in Japanese Patent Laid-Open No. Hei 4-314109, responsivity of a control system is lowered in a vibration area caused by an angle transmission error, thereby reducing vibration.
In the conventional methods, control is not applied to an accurate angle transmission error and is performed only with the disturbance torque of the reaction force of the motor 2 also containing load disturbance occurring due to change of the load 8, thus there is a limit to control performance improvement and it is also difficult to improve performance in introduction of modern control theory requiring a control target model.
That is, disturbance is all assumed to occur in an angle transmission error in the conventional angle transmission error compensation, thus disturbance occurring in an angle transmission error cannot be discriminated from disturbance occurring in a load and there is a limit to a decrease in a load tip error. Since a control design is prepared using modern control theory without clarifying (modeling) an accurate control target of the reduction gear unit 6, an accurate design based on the modern control theory requiring an accurate control target model cannot be carried out and control performance cannot be improved.